A torque converter having a lockup clutch is sometimes provided between an engine and change gear mechanism. For example, as shown in FIG. 6, this torque converter 110 has a front cover 111 connected to an input shaft (engine output shaft) 101, a pump 112 fixed to the front cover 111, a turbine 113 arranged to oppose the pump 112, and a stator 115 which is supported by a change gear case 102 through a one-way clutch 114 and arranged between the pump 112 and turbine 113 and has a torque increasing function. The inner space defined by the front cover 111 and a shell 112a of the pump 112 is filled with a working oil (fluid). The turbine 113 is driven by the pump 112 through the working fluid. A base portion 113a of the turbine 113 is spline-coupled to an output shaft 103 (turbine shaft). Rotation of the turbine 113 is output to a change gear mechanism (not shown) through the output shaft 103.
A lockup clutch 120 is arranged between the front cover 111 and turbine 113 to mechanically connect them. The lockup clutch 120 has a lockup piston 121 (to be merely referred to as a piston 121 hereinafter) which can be moved in the axial direction on the base portion 113a of the turbine 113 by a change in working hydraulic pressure (fluid pressure). The piston 121 partitions the space defined by the front cover 111 and the shell 112a of the pump 112 into a front chamber (releasing hydraulic chamber) X′ and rear chamber (locking hydraulic chamber) Y′.
A friction member 122 is attached to the outer portion of that surface of the piston 121 which opposes the front cover 111. A plurality of damper springs 123 are provided to the outer portion of that surface of the piston 121 which opposes the turbine 113. A support member 124 for the damper springs 123 is fixed to annular fixing portions 121a, which are inside the friction member 122, of the piston 121 with a plurality of rivets 125. A driven member 126 which engages with the damper springs 123 is fixed to the outer portion of that surface of the turbine 113 which opposes the piston 121.
With this structure, when a locking hydraulic pressure is supplied to the rear chamber Y′, the piston 121 is moved by the hydraulic pressure toward the front cover 111 and urged against it through the friction member 122. Consequently, the input shaft 101 and output shaft 103 are mechanically coupled (directly coupled) to each other. When a releasing hydraulic pressure is supplied to the front chamber X′, the piston 121 is moved by the hydraulic pressure to the counter front cover 111 side and separates from the front cover 111, to cancel mechanical coupling of the input shaft 101 and output shaft 103. Other than the hydraulic pressure, a centrifugal pressure generated by rotation of the working fluid acts on the front and rear chambers X′ and Y′. The higher the rotational speed (rotation count or the like) of the working fluid, the larger the centrifugal pressure. Also, the farther from the rotation center, the larger the centrifugal pressure.
In a converter state wherein the torque converter 110 is not locked, when deceleration is started, usually, the rotational speed of the output shaft 103 becomes higher than that of the input shaft 101 (engine output shaft). More specifically, the rotational speed of the front cover 111 becomes lower than that of the piston 121. Accordingly, the rotational speed of the working fluid in the front chamber X′ is influenced by the slow rotation of the front cover 111 to become lower than the rotational speed of the working fluid in the rear chamber Y′. In this case, the front and rear chambers X′ and Y′ communicate with each other through their outermost portions. Therefore, the centrifugal pressure of the working fluid in the front chamber X′ and that in the rear chamber Y′ become almost equal to each other at the outermost portions, as shown in FIG. 7, but become different from each other at portions inner than the outermost portions. More specifically, the rotational speed of the working fluid is higher in the rear chamber Y′ than in the front chamber X′. Accordingly, a change in the radial direction in centrifugal pressure of the working fluid becomes larger in the rear chamber Y′ than in the front chamber X′. Consequently, the centrifugal pressure in the front chamber X′ becomes relatively higher than that in the rear chamber Y′.
During deceleration, for example, fuel supply is interrupted to control the engine. When the fuel supply is interrupted, the lockup clutch 120 is locked to prevent an engine stall and provide an engine braking effect, and then the engine is driven by the output side.
When the lockup clutch 120 is in the converter state, if deceleration is started, on the inner side, the centrifugal pressure of the working fluid in the front chamber X′ is relatively higher than that of the working fluid in the rear chamber Y′, as described above. Thus, the piston 121 is biased to the releasing side and the lockup clutch 120 is difficult to lock. In other words, the locking response speed of the lockup clutch 120 decreases. If fuel supply is interrupted in this state, an engine stall may undesirably occur.
In view of this problem, in order to decrease the difference in centrifugal pressure of the working fluid between the front and rear chambers, for example, according to Japanese Patent Laid-Open No. 8-270751 (U.S. Pat. No. 5,613,582A and D.E.No. 4,433,256A1), an impeller member is arranged on that surface of a lockup piston which opposes a front cover. The impeller member rotates working fluid in a front chamber, so that the rotational speed of the working fluid becomes close to the rotational speed of the lockup piston.
According to Japanese Patent Laid-Open No. 11-351353, a plurality of radial ribs projecting toward a case are formed on that surface of a lockup piston which opposes a front cover (alternatively, recessed grooves are formed in the counter front cover side of the lockup piston). In Japanese Patent Laid-Open No. 11-351353, the ribs are provided to increase the pressure of the working fluid in the front chamber, so that when a clutch facing formed on that surface of the lockup piston which opposes the front cover comes into contact with the inner surface of the front cover, the facing is prevented from separating. With this arrangement, the same effect as that described in Japanese Patent Laid-Open No. 8-270751 can be obtained, although consequently.
According to Japanese Patent Laid-Open No. 58-225224, a support member for damper springs is fixed to a lockup piston with rivets. A plurality of radial ribs are formed on the lockup piston (or recessed grooves are formed in the counter front cover side of the lockup piston).
If the impeller member described in Japanese Patent Laid-Open No. 8-270751 is to be applied to the torque converter 110 having the structure shown in FIG. 6, not only the piston 121 of the lockup clutch 120 but also the structure of the torque converter 110 itself must be changed largely.
The ribs described in Japanese Patent Laid-Open No. 11-351353 can be applied to the piston 121 of the torque converter 110 without largely changing the structure or the like. To improve the locking response speed of the clutch, the ribs are desirably formed as long as possible in the radial direction, so that as much working fluid as possible can be held. In the torque converter 110, the support member 124 for the damper springs 123 is attached to the annular fixing portions 121a with the plurality of rivets 125, as described above. If the ribs extend long in the radial direction, they may undesirably interfere with the rivets 125 or fixing portions 121a. 
It is supposed that Japanese Patent Laid-Open No. 58-225224 is originally mainly aimed at reinforcement of the lockup piston. The ribs project to the counter front cover side from that surface of the lockup piston where a friction member is provided. In this case, at a portion where the ribs are formed, since the gap between the front-cover-side surface of the lockup piston and the lockup-piston-side surface of the front cover increases, the working fluid is not held sufficiently, and the working fluid between these two surfaces cannot be sufficiently rotated in the circumferential direction.